There's really not a whole lot you can do. You know those chain letters that get forwarded around the Internet? Unscrupulous people use those to harvest email addresses and send spam to them. Since they don't want to get busted for sending spam, they will often use an email address other than their own, without permission.
Unfortunately, it is exceedingly simple to send spam in someone else's name. What's to stop some snot-nosed little hacker kid from entering your email address in his email program, addressing something to 10000 people, and clicking send?
Like the others said, make sure your password hasn't been compromised. The best way to do that is to change it.
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